Microchannel plate electron multipliers (MCPs) are continuous dynodes generally consisting of glass with a high lead oxide content, made slightly conductive through a hydrogen firing process. The glass tubes forming the microchannels have electrodes at the entrance and exit. The entrance can be conical or straight, while the main section is straight, bent, or spiraled. The output current has to be a tenth or less of the strip current, otherwise the multiplier operates in a saturated mode. The amplification depends on the length-to-diameter ratio of the multiplier, the axial field strength, and the secondary-electron-emitter material.
Fabrication of microchannel plate electron multipliers and their characteristics is described by J. L. Wiza, "Microchannel Plate Detectors", NUCLEAR INSTRUMENTS AND METHODS, 162 (1979), pp. 587-601. Glass rods having a core of etchable glass and cladding of non-etchable lead glass are stacked into hexagonal arrays and drawn down to smaller size. The resulting arrays are again stacked into hexagonal arrays and redrawn to finer size, following which they are stacked and fused within a glass envelope to form a boule. The boule may be sliced orthogonally or at an angle to the boule axis, the surface of the resulting plates polished, and the core glass etched away, leaving an array of closely spaced, hollow, cylindrical microchannels in a regular geometric pattern. The plates are subjected to hydrogen reduction at elevated temperature, forming a layer of semiconductive lead on the tube interior surfaces. The front and back of the plates are then metallized to form conductive electrode surfaces.
The microchannel plates thus produced may be used singly or stacked to form multiple layer arrays. The cost of such devices is high, due to the multiple drawing and other steps required, and the useful gain limited by ion feedback and dark noise. Ion feedback is greatly increased at operating pressures above 10.sup.-6 torr, and thus operation generally requires a relatively high vacuum, 10.sup.-5 torr, and preferably 10.sup.-6 torr or below. Size is limited both by the nature of the manufacturing process as well as the necessity of providing a supporting envelope capable of withstanding the requisite internal/external pressure differential.
Thus, although such MCPs exhibit numerous advantages, they also exhibit the following problems:
a) They utilize highly complicated technology;
b) They are relatively costly to produce;
c) The sensor area is limited to approximately 125 mm.sup.2 ;
d) Their effective surface (permeability or open area) is typically on the order of only 50%-60%;
e) They have a relatively high dark noise (0.5 s.sup.-1 cm.sup.-2);
f) They can only be economically produced in a flat configuration;
g) They are made of material restricted to one type;
h) They function only in high vacuum.
All of the above constraints prevent MCPs from being used more widely, particularly in areas of medical and industrial diagnostics, flat screens, nuclear science and others.
Improvements in microchannel plate devices have centered on making them more economical to produce, or maximizing their operating parameters, for example by increasing collection efficiency, decreasing ion feedback, or increasing secondary electron emission, thus increasing gain. However, little attention has been devoted to altering the basic means of manufacture.
In Knapp, U.S. Pat. No. 4,395,437, for example, dynodes are prepared from mild steel perforated with numerous holes in a regular geometric array to form microchannels rather than use multiple glass drawings to prepare the microchannel plate. Layers of magnesium and aluminum are then formed on the dynode by evaporation of the metals at a pressure of 1 to 3.times.10.sup.-5 torr. The coating is then allowed to oxidize in ambient air and activated by heating for several hours in oxygen at pressure in the range of 10.sup.-4 to 10.sup.-5 torr. The micro-channel plates are then stacked with insulating spacers to form a multiple layer, microchannel device. No indication of gain is given, but secondary electron emissions on the order of 8 are achieved. A similar concept, using a metal/ceramic (cermet) coating is disclosed by Knapp in U.S. Pat. No. 4,099,079.
Pakswer, in U.S. Pat. No. 3,739,216, discloses both conventional high vacuum electrostatic electron multiplier tubes as well as microchannel multiplier plates where improved secondary emission at low cross-over voltage is achieved, as in Knapp '079, by the use of a cermet coating consisting of metal globules 40-500 .ANG. in size dispersed in a ceramic matrix, forming a single layer thin film having a thickness of from 200-2000 .ANG. (0.02-0.2 .mu.m). In microchannel plate devices, this film may be vacuum-sputtered or deposited by chemical vapor deposition into the tubular glass microchannels of devices such as those disclosed by Wiza. The devices suffer from the same drawbacks as those of Wiza, requiring high vacuum for operation. The metal globules lower the resistivity of the matrix.
It would be desirable to provide an electron multiplying device which is capable of more cost-effective manufacturing than microchannel plate devices currently available. It would further be desirable to provide a microchannel plate electron multiplier capable of being produced in greater variety of sizes and shapes than those presently available. It would be yet further desirable to provide a microchannel electron multiplier having high gain and low dark noise, and to provide a method for the production of such devices.
The micro random channel plate devices of the subject invention have solved many of the problems inherent in microchannel plate electron multipliers of the prior art. The subject devices provide a structure comprising randomly situated crystals, the interstices between which form randomly oriented channels for producing secondary emissions. These devices may be efficiently and inexpensively fabricated on not only flat surfaces but on surfaces of other shapes, and importantly, surfaces of widely varying size as well.